There are a number of ways to discriminate different types of heart rhythm in an implantable cardioverter defibrillator (ICD). The most obvious way to do this is to use heart rate information alone, in the manner shown for example in U.S. Pat. No. 4,475,551 to Langer et al., entitled "Arrhythmia Detection and Defibrillation System and Method", which issued Oct. 9, 1984. The purpose of discriminating different rhythms in an ICD device is to allow the device to respond appropriately to each of them. Problems occur if there are two or more rhythms which require a different response from the device but cannot easily be discriminated from one another.
The rhythms which cause most problems in existing ICD devices are atrial fibrillation (AF) and sinus tachycardia (ST). Both of these rhythms can result in a ventricular rate that is high enough for it to be considered to be ventricular tachycardia (VT). Thus, VT cannot be discriminated from AF or ST on the basis of ventricular rate alone. Additional information must be used to correctly identify these rhythms.
As discussed in an article by Arzbaecher et al., entitled "Automatic Tachycardia Recognition", appearing in PACE, May-June 1984, Volume 7 (II), pages 541-547, and in an article by Jenkins et al., entitled "Tachycardia Detection in Implantable Antitachycardia Devices", appearing in PACE, Volume 7 (II), pages 1273-1277, November-December 1984, atrial fibrillation can be readily identified by looking at the ratio of the atrial rate to the ventricular rate. This requires the use of an atrial lead, however, which may be undesirable or unavailable. Also, this method is unsuited to discrimination of ST and VT, since both rhythms may exhibit a 1:1 ratio of atrial to ventricular rate.
Timing methods have been proposed for distinguishing ST or AF from VT (see, e.g., U.S. Pat. No. 4,830,006 to Haluska et al., entitled "Implantable Cardiac Stimulator for Detection and Treatment of Ventricular Arrhythmias", which issued on May 16, 1989, and see, e.g., an article by Camm et al., entitled "Tachycardia Recognition by Implantable Electronic Devices", appearing in PACE, September-October 1987, volume 10, pages 1175-1190). These methods are of dubious accuracy and some of them also require the use of an atrial lead.
The shape or morphology of the intracardiac electrogram can be used to achieve the desired discrimination between ST and VT.
Template methods of morphology analysis provide a scalar measure of the difference between two shapes. An example of normal morphology is established, and this is compared to each heartbeat for analysis. A scalar bound or threshold on the normal variation in this scalar measure is established, and any measure that exceeds this threshold is considered to represent an abnormal beat.
A number of template methods have been heretofore described (see, e.g., U.S. Pat. No. 5,000,189 to Throne et al., entitled "Method and System for Monitoring Electrocardiographic Signals and Detecting a Pathological Cardiac Arrhythmia Such as Ventricular Tachycardia", which issued on Mar. 19, 1991, and see, e.g., an article by Throne et al., entitled "A Comparison of Four New Time-Domain Techniques for Discriminating Monomorphic Ventricular Tachycardia from Sinus Rhythm Using Ventricular Waveform Morphology", appearing in IEEE Tr. on BioMed. Eng., Vol. 38 (6), at pages 561-570, (June 1991)). Template methods in general require high sampling rates and high processing overheads. For these reasons they are unsuited for use in an ICD.
Many attempts have been made to find a single metric that can be used for rhythm discrimination purposes (see, e.g., an article by Pannizzo et al., entitled "Automatic Methods for Detection of Tachyarrhythmias by Antitachycardia Devices", appearing in the Journal of American College of Cardiology at Vol. 11, pages 308-316, February 1988). A probability density function (PDF) algorithm has been of some utility in diagnosing ventricular fibrillation (VF), but has not proven successful for discriminating ST from VT. The PDF algorithm is discussed in the aforementioned U.S. Pat. No. 4,475,551 to Langer et al., and in an article by Bach et al., entitled "Engineering Aspects of Implantable Defibrillators," appearing in a book edited by Saksena et al., entitled "Electrical Therapy for Cardiac Arrhythmias: Pacing, Antitachycardia Devices, Catheter Ablation", published by W. B. Saunders, Philadelphia, 1990, at pages 375-376.
U.S. Pat. No. 5,086,772 to Larnard et al. describes a method for combining two simple morphological features with timing information to improve the rhythm discrimination process. This method considers first the rate, and uses the morphological information only in a specified rate band, to classify individual heartbeats. The cardiac rhythm is then diagnosed on the basis of the classification of a number of successive heartbeats. The present invention is a generalization of the Larnard et al. morphological method but its application is not limited thereby.
It is, therefore, a primary object of this invention to provide an improved method of classifying heart rhythms by means of electrogram morphology.
It is another object of this invention to provide an improved method of classifying heart rhythms by means of electrogram morphology, which method is usable in an implantable cardioverter defibrillator.
It is a still further object of this invention to provide a method of classifying heart rhythms by means of electrogram morphology, which method employs a low sampling rate, requires very few multiplications per heartbeat classification, consumes very little power in the classification process, and can be implemented both in digital and analog electronics.
Further objects, features and advantages of the invention will become apparent as the following description proceeds.